The present invention relates generally to microwave signal detectors and, in particular, to a broad band microwave detector utilizing coplanar transmission lines.
In general, present microwave signal detectors for converting a microwave signal into a DC voltage for the purpose of measuring the power level of the signal are variations of a well known peak voltage detecting circuit. This circuit comprises a rectifying diode whose anode is connected to the alternating current (AC) signal to be measured, and whose cathode is connected to a by-pass capacitor which acts to filter all components of the rectified AC signal, except the direct current (DC) component. In detectors which are used at high frequencies a matching resistance is added to the input of the detector (the anode of the diode) to reduce reflected-wave effects due to impedance mismatches. The components and interconnecting configurations which operate satisfactorily at low frequencies are not useable at microwave frequencies.
For example, at microwave frequencies the rectifying diode will have a significant parallel capacitance, which represents the combined package and junction capacitance of the diode. The by-pass capacitor will have a series inductance, which represents the inductance of the electrical length of the capacitor. The terminating resistance will also have associated with it a series inductance which represents the inductance of the electrical length of the resistive termination. Additionally, a resistance is often placed in series with the diode in order to broaden the frequency response of the detector circuit. At microwave frequencies, there is an inductive component in series with this "deQing" resistance, as well as a parallel capacitance. These represent the inductance and capacitance of the electrical length of the deQing resistance.
It can thus be appreciated that the design of broad band microwave detectors involves more than the interconnection of the basic detector components.
The desirable properties of detectors of this type include: the widest possible useful frequency range, constant voltage output given a constant radio frequency (RF) power input, the lowest possible voltage standing wave ratio (VSWR) across the operating frequency band, maximum sensitivity to signal power, and minimum sensitivity to signal harmonics and DC offsets.
In the past, detector designs have been based upon the use of strip line and microstrip line technology for interconnection and fabrication. The standard strip and microstrip transmission lines are popular in the prior art because they are, mathematically, well characterized through very high frequencies. That is, the mathematical models which describe the response of such transmissions lines are well known. Additionally, strip and microstrip transmission lines are only slightly dispersive. A dispersive transmission line has an impedance which is a non-constant function of frequency. This has allowed designers to do accurate, computer aided designs that optimize the matching of the diode impedance with that of the transmission line.
A microstrip line comprises a broad ground plane and a narrow conducting strip between which is sandwiched a dielectric. The characteristics of the microstrip transmission line are a function of the conductor and ground plane areal relationships, as well as the dielectric characteristics. The electrical fields generated between the conductor and ground plane are contained wholly within the dielectric.
A strip line transmission line comprises two parallel ground planes between which is located the conductor. Sandwiched between each ground plane and the conductor is a dielectric. As with the microstrip transmission line, the characteristics of strip line transmission lines are also a function of the areal relationship between the conductor and the ground planes, as well as the dielectric. Again, the electrical fields generated between the conductor and the ground planes are confined wholly within the dielectric.
When microstrip or strip line technology is used, long ground return paths as well as generally long electrical lengths result. This is because the propagating fields are confined to the dielectric between the conductor and ground plane. Further, component placement when using such lines, often results in abrupt terminations of the transmission line, which increases parasitic problems. These long ground returns, long electrical lengths and abrupt terminations result in additional inductive and capacitive components which in turn cause the detector to have a lower resonant frequency. Low detector resonant frequency translates to lower detector bandwidths.
Designers faced with such low resonances have had to work through them, usually by placing excess inductance in series with the resistive termination at the cathode of the diode, in an effort to compensate for resonant rolloffs. However, such designs, instead of producing maximally flat responses, have resulted in varying frequency responses and have failed to significantly improve the bandwidth of the detector.
In the past, coplanar transmission lines have not been used in the construction of microwave detectors. The coplanar geometry is such that the forward and return paths of the transmission line reside on the same side of the supporting dielectric. For a coplanar transmission line, a significant fraction of the propagating fields resides outside the dielectric, thereby reducing the electrical length of the line. The coplanar line is not frequently used in the microwave art, primarily because it is not as well understood and characterized, as are the microstrip and strip line transmission lines, and because it is dispersive in nature. Additionally, its dispersive character becomes more apparent at lower frequencies than similarly dimensioned strip and microstrip transmission lines.
Contrary to the teachings of the prior art, and in accordance with the teachings of the present invention, it has been discovered that a broad band microwave detector can be fabricated using coplanar transmission line technology to provide detector frequency response bandwidths substantially greater than prior art microwave detectors, and, in general, to provide a significantly improved broad band microwave detector. Experimental results confirm a near 50% improvement in frequency response over diode detectors of the prior art. Additionally, excellent VSWR characteristics are present and fabrication of the device is greatly simplified.